1. Field of the Invention
The present invention relates to a power supply device for a vehicle, and particularly to a power supply device for a vehicle, the power supply device having two electric storage devices.
2. Description of the Background Art
In recent years, attention has been focused on an electric vehicle and a fuel-cell vehicle that use a motor in driving a wheel, and a hybrid vehicle that uses a motor and an engine in combination, and other vehicles, as an environmental-friendly vehicle.
For example, an electric vehicle requires a charging device for charging a battery. The charging device may be mounted on a vehicle, or provided at a certain location in a fixed manner.
In the case where the charging device is provided at a certain location in a fixed manner, it is necessary to move the electric vehicle to that place for charging. In other words, the charging device being provided in a fixed manner is disadvantageous in that the battery cannot be charged at a place other than the place where the charging device is provided in a fixed manner.
In contrast, if the charging device is mounted on the vehicle, there exists a problem of increase in vehicle weight. In order to solve the problem, there has been proposed a device that charges a battery from a commercial power supply at home by using a coil of a drive motor as a reactor, and by controlling a circuit element of an inverter that controls the motor. As to this device, the existing part is utilized so that the number of parts to be newly mounted is reduced, and weight increase is avoided.
Japanese Patent Laying-Open No. 08-126121 discloses a technique of preventing rotation of a rotor in an electric vehicle by allowing magnetic fields generated by three-phase coils to cancel each other out, in order to prevent movement of the vehicle caused by the rotor rotation, when the coils of the drive motor are used as a reactor to charge a battery.
If a battery in an electric vehicle as well as a hybrid vehicle can be charged from a commercial power supply at home, there may be a merit in that the frequency with which one has to go to the gas station for refueling is decreased, or may be an economical merit in a country where commercial electric power is inexpensive.
However, the Japanese Patent Laying-Open No. 08-126121 relates to an electric vehicle having two drive motors mounted on the left and right sides or the front and rear sides thereof, and hence the disclosure cannot be applied as it is to the hybrid vehicle.
Furthermore, a hybrid vehicle commercially available at present has a storage battery with small capacity, and hence even if the battery can be charged at home, an amount of energy to be supplied at that time is small. Accordingly, just after the vehicle runs for a while, a state of charge (SOC) of the storage battery decreases and the engine is required to start for activating an electric generator to generate electric power. Therefore, refueling with petrol is frequently required.
Furthermore, the hybrid vehicle performs regenerative braking in a downslope and recaptures electric power at that time to charge the storage battery. However, the capacity of the storage battery is so small that, when a running route includes a long downslope, power generated by regenerative braking is discarded as heat, once a state of charge of the storage battery reaches a prescribed value.
FIG. 14 is a diagram for describing electric power discarded as heat during regenerative braking.
Referring to FIG. 14, the storage battery is controlled such that it is used within 40-80% of the SOC range, for example, so as not to shorten its life. When the SOC of the storage battery is 60% at time t0, chargeable capacity is only 20%.
In the case of a long downslope, the storage battery is charged with regenerative electric power generated at times t0-t1. As a result, the SOC of the storage battery increases from 60% to 80%.
If the storage battery is further charged during a period from time t1 to time t2 at which the slope ends, the life of the storage battery may be shortened. The regenerative electric power generated is therefore discarded as heat. If even a part of electric power to be discarded can be recaptured, it is possible to implement a hybrid vehicle with higher energy efficiency.